1. Field of the Invention
This present invention relates generally to a method and apparatus for repairing a riser brace assembly that lends lateral support to a jet pump of a boiling water reactor.
2. Description of the Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. RPV 20 has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and is supported by a shroud support structure 26. A downcomer annulus 28 is formed between shroud 24 and sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of RPV 20 and is coupled to a jet pump assembly 34, hereafter “jet pump 34”. Jet pump 34 may include a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), and a riser pipe 38. Thermal sleeve 36 is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 is welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe 38. Riser pipe 38 extends between and substantially parallel to shroud 24 and sidewall 30.
A jet pump riser brace assembly 40 (hereafter “riser brace assembly 40”) stabilizes riser pipe 38 within RPV 20. The riser brace assembly 40 may be fabricated of type 304 stainless steel which, after periods of use, may be susceptible to cracking at welded joints. The riser brace assembly 40 is fixedly connected between shroud 24 and sidewall 30, and primarily provides lateral support to the jet pump 34 via riser pipe 38, as shown in FIG. 1. Additionally the riser brace assembly 40 is designed to accommodate for differential thermal expansion resulting from reactor start-up and heat-up, and flow induced vibration that is incumbent in the reactor water recirculation system (not shown).
FIG. 2 illustrates the riser brace assembly 40 of FIG. 1 in further detail. In FIG. 2, the riser pipe 38 has been removed for reasons of clarity. Riser brace assembly 40 primarily provides lateral support to the jet pump 34 via riser pipe 38, and includes a riser brace yoke 49 that is welded to the riser pipe 38. Riser brace yoke 49 may typically be a plate that is between about 3-4 inches thick. Riser brace yoke 49 is connected via welds to two pairs of riser brace leaves, an upper riser brace leaf (shown as 41a, 41b) and a lower riser brace leaf (shown as 42a and 42b). Leaves 41a/b and 42a/b are welded to a reactor vessel riser brace pad 130 (hereafter “reactor vessel pad 130”) which in turn is affixed to RPV sidewall 30. In an example, the reactor vessel pad 130 may be embodied as a weld buildup on the surface of RPV sidewall 30.
Thus, riser brace assembly 40 includes four riser brace leaves 41,a, 42a, 41b and 42b, which are welded at one end, shown as riser brace leaf attachment welds 143-146, to reactor vessel pads 130 provided on the RPV sidewall 30. Welds 143-146 may be commonly referred to as “RB-1” welds, for example. In the event that the structural integrity of the welds 143-146 joining the riser brace assembly 40 and the pads 130 should become degraded, a means of reinforcing or replacing the subject weld 143-146 is desired.
For example, weld failure due to vibration fatigue, and/or weld cracking due to intergranular stress corrosion cracking (IGSCC) could cause one of the welds 143-146 to fail. Separation of the riser brace assembly 40 near this weld area could adversely impact safety in BWRs. Potentially, should a riser brace assembly 40 break away from RPV 20 (e.g., at RPV sidewall 30), the riser pipe 38 becomes unstabilized, and the jet pump 34 could be adversely affected. If just one jet pump 34 is damaged, a substantial amount of piping must either be replaced or repaired.
In recent years, riser brace clamps have been fabricated and installed in a domestic BWR. These clamps are designed to provide structural support between the riser brace and an adjoining “block” structure in the riser brace assemblies of a select few BWRs. Such an exemplary clamp apparatus is described in U.S. Pat. No. 6,857,814 to the inventor, entitled “METHOD AND APPARATUS FOR REPAIRING A RISER BRACE IN NUCLEAR REACTOR”, the relevant portions of which are incorporated in their entirety by reference herein.